1. Field of the Invention
This invention relates to a field effect transistor (FET) transducer having a chemical selective system overlying the insulated gate region which has a stabilizing system on the gate surface. The transducer of this invention is stable and reliable for uses such as the measurement of concentrations of various kinds of ions, gases, enzymes, substrates, antigens, antibodies and substances which are selectively adsorbed and/or absorbed.
2. Description of the Prior Art:
There has been a continuing search for means of measurement of various chemical substances such as hydrogen ion, sodium ion, potassium ion, calcium ion, chloride ion, oxygen gas, carbon dioxide gas, carbon monoxide gas, sulfur dioxide gas, nitrogen oxide gases, various hydrocarbons and so on, not only in chemical laboratories but also in the waste water or exhaust fumes of various factories or in the exhaust fumes of car motors. Also, such measurements have become quite important in the field of medicine. Generally, it is desirable in most of those cases to carry out measurements continuously and multitudinously. For such purpose, chemical selective electrodes are employed, where it is desired to detect and/or measure electrochemically ionic concentration, redox potential, adsorption of any substances and so on.
For the measurement of ion activity or concentration, various ion electrodes have been conveniently adopted. For example, hydrogen ion, sodium ion, potassium ion, calcium ion, chloride ion, carbon dioxide, ammonia, etc. are measured according to this technique. The principle of this technique as well as the types of chemical substances measured by this method are disclosed in "Selective Ion Sensitive Electrodes", by G. J. Moody and J. D. R. Thomas (Merrow Publishing Co., Ltd., Watford, England, 1971). Thus, a glass electrode is a convenient tool for continuous measurements of the concentrations of the various chemical substances mentioned above, using various selective chemical sensitive membranes. Such electrodes, however, are limited in the following ways. (1) When the resistance of a glass membrane is as high as about 10 M ohm, amplifiers designed to work with such an electrode must possess high input impedance. (2) The mechanical strength of such electrodes is very low. (3) When such an electrode is miniaturized, high membrane resistance resulting from the small area of the glass membrane brings about difficulty in electric insulation and, therefore, poor stability in its operation. (4) Integration of such an electrode for multiple channel purposes results in impractical bulkiness.
Other prior art apparatus disclosed in "Development, Operation and Applications of the Ion-Sensitive Field Effect Transistor as a Tool for Electrophysiology" by Piet Bergveld, IEEE Transactions of Biomedical Engineering, 1972, 342, includes a new type of electrode where a semiconductor is employed and those limitations found in the above-mentioned electrodes are avoided. Bergveld indicated therein the measurement of hydrogen and sodium ion activities in aqueous solution with the use of a metal oxide semiconductor field effect transistor (MOSFET) modified by removal of the gate metal, i.e., an insulated gate FET having silicon oxide as the gate insulating layer on its gate region. This FET transducer has several excellent practical advantages; it is feasible for super-miniaturization without influencing output impedance as well as for integration of various transducers where multiple sensors are prepared on one tiny silicon piece. One problem with the device of Bergveld is that the insulated gate layer comprises silicon oxide and therefore the immersion of the transducer in an aqueous solution results in continuation of the hydration process of the silicon oxide insulation layer. This, of course, affects the accuracy of the ion activity measurement and may also result in electrical leakage of the device.
Those limitations found in the device of Bergveld have been overcome by T. Matsuo and K. D. Wise, as disclosed in "An Integrated Field Effect Electrode for Biopotential Recording", IEEE Transactions on Biomedical Engineering BME-21, 1974, 485. The device disclosed therein comprises a silicon nitride layer overlying a silicon oxide layer in the insulated gate region, the latter being the same as that disclosed by Berveld. This structure of the insulated gate layer results in the prevention of the hydration process of the silicon oxide layer and therefore enables a stable measurement of hydrogen ion activity. Moreover, Matsuo and Wise suggested therein that their device would be further extended to transducers sensitive to various chemical substances, although they did not describe a specific structure.
A device designed to measure various chemical substances is disclosed in U.S. Pat. No. 4,020,830. The device described therein comprises selective chemical sensitive systems, similar to those of the glass electrode, overlying the insulated system of the gate region. Thus, the application of various selective chemical sensitive layers on the insulated gate region enabled the FET transducers to detect or measure selectively antigens, antibodies, hormones, enzymes, reducible gases, and the like, as well as various ions. Seemingly, the devices described therein are promising as sensors of the future as they are usable to measure various chemical substances by applying various selective chemical systems sensitive to any substances of interest on the insulated gate region. Also, the amplifiers to be utilized with the devices are rather simple in structure and are manufactured inexpensively when compared with those needed for glass electrodes. Also, they have advantages such as feasibility of super-miniaturization, mass-production, etc., which are inherent features of transistors.
It was, however, discovered by the present inventors that such FET transducers did not provide stable and reliable measurements and were not sufficiently convenient for practical use for various reasons. First of all, when a non-selective interaction such as adsorption or precipitation on the surface of the gate region takes place, it often disturbs its output signal, often by as much as 1-2 units on the pH scale. For example, non-selective interactions were found to disturb the measurement of FET transducers in factory waste water and in body fluid due to non-selective precipitation of floating micro-particles and adsorption of protein on the gate surface, respectively. This resulted in many problems such as instability of measurement, signal drift, etc., when used for long periods. Secondly, FET transducers are sensitive to light, yielding electromotive force with light-illumination, which results in experimental error, electronic noise, and therefore less accurate measurement. This problem, of course, can be avoided when the transducer is used in a dark room or with a dark cover to shut out light. It is obvious that such a procedure is annoying and impractical for a general purpose transducer. The present invention is directed to overcoming the above-mentioned problems which are important for practical use and, as a result, an FET transducer is provided that permits stable and reliable measurement. Thus, the present invention made it feasible to put FET transducers to practical use, in the monitoring of factory waste water and exhaust fumes, in various industrial instruments and in laboratory experiments, as well as in measurement and monitoring of chemical constituents contained in blood, cerebrospinal fluid, tears, urine, lymph, and the like. In the latter types of measurement, the small size and disposability (ease of massproduction) of FET transducers made possible by photo-engraving techniques make them most advantageous compared to conventional measuring devices.